Galling is a condition that occurs as a result of friction between two metal surfaces, which may be found, for example, at mating surfaces of two components in sliding contact. Heat generated by friction can cause localized welding and metal transfer between the metal surfaces. This localized welding and metal transfer has the effect of roughening the surface topography of the contacting metal surfaces. The roughened surfaces cause even more friction and contribute to further galling, which can ultimately cause unacceptable performance degradation or failure of the sliding components.
One way to impart added wear resistance to sliding surfaces of mating components may include increasing the surface hardness of the sliding components. While such an increase in hardness may increase wear resistance of the components, harder materials may lack sufficient lubricity to effectively reduce friction between the sliding surfaces. Thus, galling may still occur.
In certain applications, galling may be minimized or avoided by adding a lubricious layer of relatively soft metal between sliding surfaces of the mating components. Merely adding a soft metal layer to the sliding surfaces, however, may be inappropriate for certain applications. For example, many sliding components operate in harsh environments that may include high temperatures and high bearing loads. Thus, for certain applications, a gall-resistant coating may be required to exhibit a combination of properties including high lubricity and sufficient hardness to withstand a particular set of environmental conditions. Generating such a coating can be challenging and may include the use of multiple constituents to provide a desired combination of physical properties.
U.S. Pat. No. 6,544,597 to Takahashi et al. (the '597 patent) describes a method and apparatus for generating a coating for a sliding component using a combination of materials. Specifically, the '597 patent describes a mixed powder plasma spraying technique in which both an iron based powder and an aluminum based powder are separately fed to a plasma spray apparatus. In the method of the '597 patent, these powders are melted in the plasma spray and deposited together as a surface coating.
While the method of the '597 patent may potentially produce adequate gall-resistant coatings, this method has several shortcomings. For example, providing the iron and aluminum based constituents separately to the plasma spray apparatus requires a complicated scheme for controlling the precise ratios and feed rates of the different materials needed to generate a desired coating. Also, the plasma spray technique of the '597 patent tends to produce coatings with low densities and high levels of oxidation, which may result in brittle coatings. Further, the coatings made by the plasma spray method of the '597 patent can suffer from low bond strength due to high porosity and tensile residual stresses in the plasma-spray-deposited coatings.
The disclosed coating and method are directed to overcoming one or more of the problems set forth above.